FORMATION OF INDIGO- BLUE. 195 



when the latter has begun to change. Of the two colouring 

 matters, the indigo-blue is always the first to be formed. If 

 sulphuric or muriatic acid be added to a solution of indican 

 in the cold, and the mixture be allowed to stand in the cold 

 for some time, a slight precipitate is gradually deposited 

 which consists almost entirely of indigo-blue. It is only 

 after boiling for some time that the formation of indirubine 

 commences, when the colour of the liquid changes from blue 

 to purple. The constant occurrence of a red colouring 

 matter both in indigo and in the indigo-bearing plants along 

 with indigo-blue, a fact which has been repeatedly observed, 

 has led chemists to suspect that there must be some neces- 

 sary connection bet«veen the two. From the experiments 

 just described, it follows that in the case of woad at least 

 they are both products of decomposition of one substance. 

 If nitric acid be added to a watery solution of indican, a 

 slight deposit of indigo-blue is formed, which of course dis- 

 appears immediately on heating the liquid. 



There is another very remarkable property of indican, 

 which I have to describe, a property, the knowledge of 

 which will probably throw great light on the process of ma- 

 nufacturing indigo. If indican in the form of syrup, as 

 obtained by evaporation of the watery solution, be heated 

 for some time in the waterbath, or if its watery solution be 

 boiled, or even moderately heated, it undergoes a complete 

 metamorphosis. If the solution be now evaporated it leaves 

 a yellow syrupy residue, not to be distinguished in appear- 

 ance from indican itself. It will be found, however, to have 

 become insoluble in ether, and not easily soluble in alcohol. 

 If ether be added to its solution in alcohol, the solution be- 

 comes milky and deposits oily drops which collect at the 

 bottom of the vessel to a yellow or brown syrup, the un- 

 changed indican, if there be any present, remaining dissolved 

 in the ether. In its other outward properties, it has not 

 undergone any marked change. In the next stage of the 



